Is this guy a conspiracy theorist because he certainly sounds like one? If so then there really is no point in arguing with them as no amount of contrary evidence will ever be good enough. Conspiracy theorist will always find a way to claim the evidence was faked.
You can put a flat earther in a rocket, blast him into space, show him the curvature of the planet, bring him back safely and he will still shout "FAKE!" He'd probably claim that the windows on the rocket were actually monitors and it was all CGI or something along those lines.
The point is, their cognitive dissonance is so strong that it is impossible to reason with these people. Don't bother wasting your breath with your colleague.
